In the art of surfactants, it has been heretofore proposed that mild amphoteric surfactants could be prepared incorporating the imidazoline nucleus and containing quaternary ammonium complexes. An examination of commercially available surfactants alleged to be of this general type reveals that characteristically such are in the nature of complex physical mixtures which can have somewhat variable properties and which can contain minor components that can cause undesirable effects, such as skin and eye irritation, turbidity, poor storage characteristics, and the like.
Amphoteric surfactants derived from imidazoline have always heretofore been described as containing cyclic quaternary ammonium compounds. It is known in the art that quaternary ammonium compounds as a class of compounds tend to be skin and eye irritants. Thus for instance Arno Kluge, in an article on "The Properties of Quaternary Ammonium Salts and their Use in Cosmetic Products" published in Parfuemerie and Kosmetik 42, 341-6 (1961) describes alkylaromatic quaternary ammonium salts with alkyl groups having a chain length of from C.sub.12 to C.sub.16 as being skin irritants. Similarly J. M. Quack, in his article on "Quaternary Ammonium Compounds in Cosmetics" published in Cosmetics and Toiletries, vol. 91, pages 35-52, of February 1976, mentions that, although varying in their degree of toxicity and dermatologic insult depending on the alkyl chain length, quaternary ammonium compounds are classified in the literature of the art as being skin irritants.
Therefore, it would be desirable to have amphoteric surfactants derived from C.sub.12 -C.sub.18 substituted imidazolines which are substantially free from quaternary ammonium salts. However, it is believed to be very difficult and probably impossible to make such amphoteric surfactants by following the prior art teachings. Thus for instance U.S. Pat. Nos. 2,781,354; 2,781,355; and 2,781,356 teach a method for making ampholytes from substituted imidazolines in which the end product is described as being primarily a cyclic quaternary ammonium hydroxide.
It has now been discovered that reactions of imidazoline derivatives in aqueous medium with neutral salts of chloroacetic acid and in absence of strong alkali form a cyclic betaine which is stable to hydrolysis at a pH below about 9. This cyclic betaine is a powerful surfactant which solubilizes the as yet unreacted alkyl imidazoline into the water layer. As a result of this dissolution the imidazoline ring opens up and straight chain amides are obtained which in turn react with the as yet unreacted chloroacetic salt to form amino acids which bring the pH further down, a property of these reaction conditions which protects the cyclic betaine from hydrolysis.
It has further been discovered that upon subsequent treatment with a strong alkali a straight chain compound is obtained from the cyclic betaine, said compound having a tertiary (trisubstituted) amide, in which one such substituent is a carboxy methyl group. This compound imparts what are now believed to be synergistic and even unique properties to the mixture of amphoteric compounds coexisting in a final product of this invention. Such properties are now believed to include one or more of enhanced solubility, foamability, and mildness. We were unable to obtain such a combination of properties either by the synthesis of ampholytes from the precursor amide directly, or by preparing ampholytes from imidazoline by using the currently proposed methods as shown in the prior art.
Product compositions of this invention prepared as herein described are clear solutions, yellow straw in color.
A further and very significant feature of this invention is the circumstance that, if the process step sequence of this invention is not followed (for example, if the first and second process steps are not sequentially practiced, but are instead simultaneously practiced), the storage of stable product compositions of this invention are not achieved and clouding upon storage of such other composition is characteristically observed.
There is a need in the art for amphoteric surfactants which are derived from substituted imidazolines, which are highly foamable (as can be considered characteristic of such amphoteric surfactants), which have minimal skin irritating and eye irritating characteristics, and wherein a carboxymethyl or a hydroxyethyl group is attached to the amide nitrogen (making tertiary amide compounds). Concurrently, of course, methods for making such improved amphoteric surfactants are likewise needed.